Pretty sure the login for article comments is the same as it is for the forums.
They are different. People with accounts on the forums will need to sign up on the main page before being able to post comments there.
Just try using your username and password there, you will get a "Login details are incorrect" message.
Weird I do use my username & password here as I do there. But I seem to have been a member for so long that I can't remember creating a seperate account with identical credentials.